The weird thing was that the error only happened when publishing the particular file via the publish queue using the 'run-periodic-tasks' script, but it didn't happen when publishing the file 'by hand'.

Six Apart has announced their first event outside of Japan in a long time, the Movable Type Idea Exchange in New York City on October 18th. I'm certainly going to be there. Admission is free but you have to RSVP. Anyone else going? Let me know in the comments.